The present invention is a method of making a nonwoven fabric-like product using bacterially formed cellulose as a binding agent. The invention further includes the wet formed products made by the above method. A particularly useful bacterial cellulose is one formed in aerated agitated culture using a microorganism of the genus Acetobacter genetically selected for cellulose production under agitated conditions.
It has been known for many years that cellulose can be synthesized by certain bacteria, particularly those of the genus Acetobacter. This phenomenon has received considerable study in the years following 1930. While a great deal has been learned about the production of bacterial cellulose in the past 50 years, it has been primarily of academic interest and only very recently has it been proposed for use in products having potential commercial significance.
Taxonomists have been unable to agree upon a consistent classification of the cellulose producing species of Acetobacter. For example, the cellulose producing microorganisms listed in the 15th Edition of the Catalog of the American Type Culture Collection under accession numbers 10245, 10821, and 23769 are classified both as Acetobacter aceti subsp. xylinum and as Acetobacter pasteurianus. For the purposes of the present invention any species or variety of bacterium within the genus Acetobacter that will produce cellulose under agitated conditions should be regarded as a suitable cellulose producer for the purposes of the present invention.
Acetobacter aceti subsp. xylinum, for purposes of brevity hereafter referred to simply as Acetobacter xylinum, is normally cultured under static conditions with the cellulose microfibrils being produced at the air medium interface. Most bacteria of this species are very poor cellulose producers when grown in agitated culture. Various reasons have been proposed to account for this phenomenon. It may be that agitated culture induces a tendency to mutation to noncellulose producing types. Perhaps for this reason two recent proposed products using bacterial cellulose have been based on static cultures of Acetobacter xylinum.
Brown, in U.S. Pat. No. 4,378,431, describes a method for imparting hydrophilic characteristics to fibrous hydrophobic substances. The Acetobacter bacterium is cultured in the presence of the synthetic substrate. During the culture process, cellulose microfibrils are produced on and around the surface of the substrate. The substrate is preferably in the form of a fiber which may be either woven or nonwoven. Natural substrates may also be used. The inventor notes, without providing an example, that the strength of paper may be increased. The product is seen as a combination of a bacterial pellicle and the substrate. To the present inventors' knowledge this process has not had any commercial utilization due to several problems. A major one is that the substrate must be retained in the culture medium for a long enough period of time for cellulose to grow on the surface. For most proposed applications further treatment, such as a hot caustic leaching, would then be necessary to remove the residual bacteria before use. Such a treatment could seriously damage the integrity of the substrate.
U.K. Patent application 2,131,701A describes formation of bacterial pellicle which is ultimately said to be usable as a wound dressing. This process also employs static culture conditions for formation of the cellulose pellicle.
In our earlier U.S. patent application, Ser. No. 788,915, filed Oct. 18, 1985, we disclose Acetobacter varieties which are vigorous cellulose producers under agitated culture conditions. The cellulose produced by the microorganisms and culture conditions disclosed in this application appears to be a unique type, physically quite different from the bacterial cellulose produced in static cultures. It has a highly branched, three dimensional, reticulated structure. A normal cellulose pellicle produced in static culture tends to have a lamellar structure with a very much lower amount of branching. This application claims sheets formed from the bacterial cellulose. These sheets are unique in their very low air porosity and high resistance to densification by conventional means.
The present invention is not directed to sheet materials in which bacterial cellulose is a major component. Rather, it is directed to the use of bacterial cellulose as a minor component used as a binder for other fibers in wet formed sheets.